High quality gears such as spur gears for aerospace applications are required to have hardened gear tooth surfaces to minimize wear, with the interior portion of the gear tooth remaining unhardened to prevent the gear from being brittle, shock-susceptible, and subject to breakage. Typically, the industrial process for manufacture of high quality gears requires either case carburizing and hardening, or induction hardening, of the gear teeth to a specified contour, case depth, and hardness.
Carburizing, which introduces carbon into the surface layer of a low-carbon steel by heating the gear in a furnace while it is in contact with a carbonaceous material to diffuse a portion of the carbon into the steel from the surface, converts the outer layer of the gear into high-carbon steel. The gear may then be removed from the furnace, allowed to cool, and heat-treated by being brought to a high temperature above the transformation point and quickly quenched, transforming the high-carbon surface layer into a hard case containing martensite, while leaving the low-carbon core tough and shock-resistant. Quenching involves rapidly cooling the heated surfaces either conventionally by a gas or a liquid, or by the heat sink effect of the gear's mass (not possible where the gear is heated in a furnace).
Carburizing requires selective masking of the gear, as well as subsequent chemical mask removal, to prevent surface portions of the gear which must remain non-hardened from being hardened in the carburizing process. The quenching step also produces distortion in the part, which will then invariably require a final grinding operation to correct the distortion, particularly in those gears destined for use in aerospace applications and which are required to be of extremely high quality and have critical tolerances.
Quenching dies may be used to minimize distortion during the quenching operation by placing the heated gear into a quenching die fitting the part perfectly. The quenching operation is then performed, and the part may be removed from the quenching die.
It may be appreciated that the carburizing method of hardening gears is both energy and labor intensive, and is therefore quite expensive. In addition, carburizing is quite time-consuming and requires a large amount of equipment, including a furnace, quenching dies which must be custom made for each gear being manufactured, masking equipment, and regrinding equipment.
One alternative to carburizing is induction hardening, where the gear to be hardened is placed inside a coil through which a rapidly alternating current is flowing. Heat is rapidly generated within localized portions of the gear by electromagnetic induction, with the depth of the case being controlled by the frequency of the current in the coil. The gear is then quenched, and induction hardening thus also presents the problem of distortion in the gear which may subsequently require final regrinding operations. As such, induction hardening is also expensive and time-consuming.
Industrial lasers have shown promise in selective rapid heating of surfaces to be hardened. The surface to be heated by a laser beam is generally prepared by applying an absorptive coating which aids in energy transfer from the laser beam into heat energy within the part. One advantage of using a laser to quickly heat a surface is that conventional quenching by a gas or a liquid is unnecessary since only a shallow surface area of the part is heated. The part will, therefore, actually self-quench, due to the extremely high heat differential between the shallow surface area heated by the laser and the bulk of the part being processed.
Attempts have been made in the past to use industrial lasers for surface heat treatment of parts such as gears, and two such attempts are described in U.S. Pat. Nos. 4,250,372 and 4,250,374, both to Tani. The '374 patent describes the technique of gear hardening using a single beam, and '372 patent describes a technique using two or more beams to obtain more even heating of the gear tooth areas to be hardened.
These patents are both largely impractical for several reasons. First, using the techniques taught in the Tani patents, it is virtually impossible to get an even case depth in the V-shaped area including the flank or side of one gear tooth, the flank of a second adjacent gear tooth, and the root area between the two gear teeth. Laser beams do not have uniform energy density except where they are focused to pinpoint precision, and the more widely focused laser beams of the Tani patents have "hot spots" in the beams resulting in unpredictable and non-uniform heating of the gear surface. Even by using sophisticated lens technology to vary the energy density of the laser beam or beams used, the case depth will not be of sufficient uniformity to meet the specifications for aerospace components. Another problem encountered in using the techniques taught by the Tani patents is that the edges of the gears are frequently burned or melted away to some degree, making the repeatability of any type of quality standard extremely difficult.
Another problem present in the art is back-temper, in which a surface already hardened is reheated and softened by the hardening process of a second surface, in this case an adjacent gear tooth or V-shaped area. Since the Tani patents harden one flank of the gear tooth in one operation, and the opposite flank of a gear tooth in a second operation, sufficient heat is generated in the gear tooth when the second flank is hardened to substantially diminish the hardness in a portion of the first flank in all but very coarse gears. Thus, it may be appreciated that the Tani patents do not present a viable alternative to carburizing and hardening of gears for aerospace or other critical applications.
A more successful technique is taught by U.S. patent application Ser. No. 509,530, filed June 29, 1983 by Benedict and assigned to the assignee of the present application, which application is hereby incorporated herein by reference. The Benedict application splits a laser beam into two identical beams which are focused and directed onto opposite working surfaces of a workpiece such as a gear tooth to simultaneously harden both working surfaces, thereby preventing back-temper. This technique is highly successful for hardening of teeth in lightly loaded gears running in one or both directions, but its shortfall is that the root area between adjacent gear teeth is not hardened. While the root area of a gear is not needed as a wear surface, it is critical in highly loaded gears since it will, if hardened, prevent gear teeth from bending (bending deflection) under heavy load since the hardening of the root area causes the teeth of the gear to be stiffened up while leaving the interior surface of the gear softer for shock-resistance. It may, therefore, be appreciated that a technique for hardening the entire V-shaped groove between two adjacent gear teeth without causing back-temper in surfaces previously hardened must be achieved to make viable laser gear hardening of heavily loaded, high quality gears.